In the current mobile telecommunication, each mobile agent, for example, edge router broadcasts the Agent Advertisement (AA) in its subnet, therefore, the Foreign Agent Care-of-Address (FACOA), which is allocated by MA and is a care of address (COA) corresponding to the MA, is informed to each user equipment (UE) in the subnet, so as to support the necessary service handover. However, since the UE may, during the service transmission, move from its home subnet to another subnet, hereinafter referred to as foreign subnet, if the UE waits for the broadcasted AA passively, the service may be interrupted due to the long time waiting.
In order to solve the problem, the UE may request the mobile agent in the current subnet to send AA by actively sending the agent solicitation (AS) message, which is an ICMP (Internet Control Message Protocol) router discovery message. For the UE, the mobile agent in the home subnet is called home agent, and HA for short; while the mobile agent in the foreign subnet is called foreign agent, and FA for short. Besides, the UE may register the relationship between the care of address that it uses, for example FACOA or COCOA, and its permanent address, for example, HOA (home address), to its HA. Therefore, when the counterpart node (CN) communicates with the UE, the CN first sends the IP packet to the HA of the UE, then the HA sends the IP packet to the corresponding FA via the tunnel in network layer, for example, the HA may encapsulates the IP packet based on IP protocol. After receiving the IP packet, the FA carries out the corresponding de-capsulation on the received packet, and forwards the packet to the UE according to the UE's address carried in the IP packet.
One subnet usually comprises multiple mobile agents, when the UE does not know the unicast address of any of the FAs in the current subnet, it is not feasible to send the AS in the unicast packet form. To this end, in the prior art, a specific multicast address is allocated for all the mobile agents in one subnet. When the UE moves into any subnet, as long as the AS message it sends carries the multicast address, access device can identify that the AS message is destined to all the mobile agents in the subnet.
With the further study in the fix and mobile network convergence (FMC), the mobile IPv4 service will be deployed in the fixed access network. However, since the operator prohibits the UE to send multicast IP packet from the economic and security point of view, the access devices, such as DSLAM, discard the IP packet whose multicast source is UE. Therefore, when the UE sends the AS in the form of multicast IP packet, the AS request message can not reach any mobile agent in the current subnet, thus cannot trigger the sending of AA, so that the service may be interrupted. For the same reason, RRQ message sent by the UE as multicast IP packet cannot reach any mobile agent either, so that the registration cannot be completed on time.